I'm not sure if there is a specific brand available, but I'm looking for 4x5 digital back. I've seen them on other camera. I've been told old stories how photographers that first used these cameras fixed things with chewing gum and shoe strings, so I figured someone would have thought of how to attaach a digital back on a speed graphic 4x5.

It would be perfectly possible to mount a digital back on a 5x4 Speed Graphic, and it would work, i.e. correct focus distance etc., as long as the camera does not have a Graflex back. Some types of digital back can be used on cameras with a Graphic back, If you're lucky, and your camera has a Graflok back then you can use just about anything. That's one of the great things about large format, you can use just about anything on the front and back of any camera made in the last fifty years or so.

A couple of problems with the digital back, if it is of the scanning type, which produce the best image quality, then it can take several minutes to make an exposure, and it will probably cost about 100 times as much as the camera is worth.

I use 4x5 transparencies and scan them on a Microtek 6800 scanner. I used to use an Acer 620PT, but it didn't work when my wife upgraded her computer so I had to get a new one. I easily get 20-80MB files out of the 4x5's with Ektar 203 and other lenses.